Because of their strength, heat resistance, and chemical resistance, polyester containers, films, sheets, and fibers are used worldwide in numerous consumer products. In this regard, most commercial polyester used for polyester containers, films, sheets, and fibers is polyethylene terephthalate polyester.
Polyester resins, especially polyethylene terephthalate and its copolyesters, are also widely used to produce rigid packaging, such as two-liter soft drink containers. Two-liter bottles and other polyester packaging produced by stretch-blow molding possess outstanding strength and shatter resistance, and have excellent gas barrier and organoleptic properties as well. Consequently, polyethylene terephthalate and other lightweight plastics have virtually replaced glass in packaging numerous consumer products (e.g., carbonated soft drinks, fruit juices, and peanut butter).
In a conventional process for making polyester resins, modified polyethylene terephthalate is polymerized in the melt phase to an intrinsic viscosity of about 0.6 dL/g, whereupon it is further polymerized in the solid phase to achieve an intrinsic viscosity that better promotes article formation. Thereafter, the polyethylene terephthalate may be formed into articles, such as by injection molding preforms, which in turn may be stretch-blow molded into bottles.
Conventional polycondensation processes that include solid state polymerization (SSP) can suffer from poor performance, in part because of poor reaction rates during solid state polymerization. Traditional metal catalysts may not satisfactorily increase the SSP reaction rates. Slower SSP rates require longer reaction times to achieve the desired polymer molecular weight, which in turn increases production costs.
It would therefore be desirable to develop a catalyst system and associated methods for improving the reaction rates during solid state polymerization of polycondensation polymers, particularly polyethylene terephthalate.